PROJECT SUMMARY/ABSTRACT Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) represent a spectrum of devastating neurodegenerative diseases characterized by progressive impairment in cognition, emotion, and/or motor function. There are no effective treatments at present, although clinical trials are in process and being planned. Some forms of FTD and ALS are inherited in an autosomal dominant manner as a result of a mutation in one of several genes. The major gene that links familial FTD and ALS is C9orf72, which was identified in 2011 and in which the mutation is an expansion of a hexanucleotide (G4C2) repeat sequence. The mechanism(s) by which this mutation results in neurodegeneration is currently unclear. C9orf72 ALS/FTD is one of over thirty neurodegenerative diseases caused by the expansion of a short repetitive repeat tract (microsatellite repeat). In general, these repeats tend to be unstable, changing in length both across generations and over time in somatic cells, with considerable evidence that changes in somatic repeat length can modify disease phenotype. Notably, the C9orf72 repeat is highly somatically unstable. DNA repair genes play critical roles in determining the instability of microsatellite repeats; most notably, genes encoding mismatch repair proteins modify the instability of several different disease-associated repeats (CAG/CTG, CGG and GAA), indicating a fundamental role for these proteins in regulating the length of diverse types of repeat tracts, and signaling common mechanisms of disease modification. Here, we will carry out studies to explore factors contributing to C9orf72 G4C2 repeat dynamics in mice and in patient-derived neurons, investigating DNA repair genes as well as other candidate genes that control aspects of DNA and chromatin structure that have been implicated in C9orf72 pathogenesis. Using C9orf72 BAC transgenic mice expressing Cas9 we will use adeno-associated viruses (AAVs) to deliver single guide RNAs (sgRNAs) to inactivate genes of interest in the brain and periphery, and will analyze G4C2 repeat length over time in different tissues. As a parallel and complementary approach, we will use CRISPR/Cas9 technology to inactivate genes of interest in patient-derived induced pluripotent stem cells (iPSCs) harboring C9orf72 expansions, and will determine G4C2 repeat length during iPSC passaging and upon differentiation to both cortical and motor neurons. Together, these studies will provide novel insight into factors underlying C9orf72 repeat dynamics and explore mechanisms of disease modification that are common across microsatellite expansion disorders, with the ultimate goal of identifying therapeutics targeting the causative mutation itself. .